The present invention relates to a gearshift for bicycles.
As known, a gearshift for bicycles usually comprises a first element, or "upper body", pivoted on the bicycle frame and connected by means of two connecting rods to a second element, or "lower body". On the lower body is pivotally mounted a rocker arm which, by means of two wheels and a spring mechanism, performs the functions of guding and stretching the chain. The assembly comprising the upper and lower bodies and the two connecting rods forms a parallelogram mechanism which, controlled by a flexible driving cable opposed by a spring, shifts the rocker arm in a direction usually parallel to the rear wheel axis, thereby guiding the chain in correspondence of the sprocket selected each time among the sprockets of the freewheel device of said rear wheel.
The device described shows some defects in its conventional forms of construction: the motion of the wheels guiding the chain in correspondence of the sprockets--which is substantially parallel to the axis of the sprocket assembly--leads such wheels to operate at a short distance from the large sprockets and at a far longer distance from the small sprockets; the motion of the rocker arm--which is also parallel to said axis--moreover causes the chain to wind around each sprocket with very similar winding angles, with the risk of having too few teeth meshing with the small sprockets, thereby making it more difficult to recover the chain.
A solution to these problems has been seeked in the past through various systems, the ones most adopted being that of offsetting the aforementioned chain guiding wheel in respect of the rocker arm rotation axis so as to draw it close to the sprocket assembly (with the drawback, however, that its position becomes too much influenced by the diameter of the pedal crank gear), or that of inclining the oscillation axes of the connecting rod of the articulated parallelogram, so that the trajectory of the rocker arm becomes skew in respect of the rotation axis of the sprockets (with a certain advantage for what concerns the first of the cited defects, but without eliminating the second).
More recently (see European Pat. Nos. 0013136 and 0036317), it has been proposed--also with the object of reducing the aforementioned drawbacks--to introduce into the gearshift means causing rotations of said upper body in association to the oscillations of the articulated parallelogram. Nevertheless, the means so far provided for this purpose have not allowed, on one hand, to univocally define the position of the chain guiding wheel for each sprocket meshing with the chain and, on the other hand, to easily adapt the gearshift to any type of sprocket assembly, with a continuous and congruent control of the oscillations of the articulated parallelogram and of the rotations of the upper body of the gearshift.